Newsgroups: sci.space.tech
From: henry@spsystems.net (Henry Spencer)
Subject: Re: 2 fuels, 1 oxydizer SSTO...
Date: Sat, 16 Dec 2000 19:37:41 GMT
In article <20001215040526.13767.00002154@ng-mk1.aol.com>,
Doug Goncz <dgoncz@aol.comm> wrote:
>Two tanks sitting next to each other weigh a lot more than one tank of twice
>either's capacity...
Rocket tanks tend to be dominated by pressure loads, and the mass of a
pressure vessel is proportional to its volume, not its surface area
(because larger tanks need thicker walls to carry the same pressure
loads). So in fact the tank mass is the same, regardless of whether it's
all in one piece or subdivided. What you do have to allow for, mind you,
is the extra structure needed to tie subdivided tanks together.
>Adjacent tankage of methane with liquid oxygen is practical. The fluids, while
>chemically very different, have nearly the same cryogenic properties...
An interesting alternative is LOX and propane. Propane is still liquid at
LOX temperatures, and is rather denser than methane.
--
When failure is not an option, success | Henry Spencer henry@spsystems.net
can get expensive. -- Peter Stibrany | (aka henry@zoo.toronto.edu)

Newsgroups: sci.space.tech
From: henry@spsystems.net (Henry Spencer)
Subject: Re: 2 fuels, 1 oxydizer SSTO...
Date: Mon, 18 Dec 2000 20:53:22 GMT
In article <20001218121124.08122.00004891@ng-mi1.aol.com>,
Doug Goncz <dgoncz@aol.comm> wrote:
>>Rocket tanks tend to be dominated by pressure loads, and the mass of a
>>pressure vessel is proportional to its volume, not its surface area...
>
>Heck, now I'm totally confused. There are no pressure loads in crytanks, are
>there? They're at ambient.
No, in general not. Even in pump-fed rockets, a certain minimum tank
pressure is usually needed to suppress cavitation in the pumps -- a
problem that is particularly bad for cryogenics because they're invariably
at or just below their boiling point. The exact requirement depends on
pump design, but it is usually significantly above ambient, even at sea
level.
This also has some useful side effects, like adding structural strength to
the tanks. (Atlas and Centaur are the only stages which rely on this on
the pad, but almost everybody relies on it -- to at least some extent --
for flight.)
>The volume of a unit volume sphere is one, and we'll call the corresponding
>surface one. Two such spheres have volume two and surface two. But a sphere of
>size cube root of two or 1.26 has volume two and surface 1.59, a savings of .41
>over the two required with separate tanks. That's a 21 percent weight saving!
Again, you are assuming that tank mass is proportional to surface area,
which in general it is not. Some things, like any insulation required, do
scale with surface area... but pressure-vessel strength members do not.
>I do agree that larger pressure tanks need thicker walls to carry the same
>pressure contents. Hoop stress and all that, right?
Take any plane that slices through the pressure vessel, and consider the
problem of trying to prevent the vessel from separating at that plane.
The pressure load trying to cause such a separation is proportional to the
surface area of the slice. The wall length available to resist it is the
outer edge of the slice. The load scales with the square of size, the
wall length only linearly... so as the size goes up, the wall thickness
must grow as well. Chase it through the math, and wall mass is simply
proportional to volume.
--
When failure is not an option, success | Henry Spencer henry@spsystems.net
can get expensive. -- Peter Stibrany | (aka henry@zoo.toronto.edu)

Newsgroups: sci.space.tech
From: henry@spsystems.net (Henry Spencer)
Subject: Re: 2 fuels, 1 oxydizer SSTO...
Date: Tue, 19 Dec 2000 13:08:09 GMT
In article <20001218172343.07564.00010719@ng-cs1.aol.com>,
Doug Goncz <dgoncz@aol.comm> wrote:
>Oops. Big tanks have fluid head at the bottom and are most likely designed to
>be the same all around for simiplicity.
Actually, it's common to vary the wall thickness over the height of the
tank. Those structures often are not as simple as they look.
Hydraulic head is significant in big tanks, but supplying the minimum
inlet pressure of the pumps usually matters more.
>Might inclusion of kerosene in the boosters allow enough saving of oxygen
>burning with hydrogen to make a larger oxygen tank unnecessary? The oxygen
>saved by not burning the SSMEs at full throttle and maximum thrust mixture
>would be burned with kerosene in liquid boosters. Ideally, no booster oxygen
>tanks would be required.
The usually-preferred approach is the other way 'round: the boosters feed
propellants to the core engines until booster cutoff. Tank mass in the
core has to be carried all the way to orbit (or almost, in the case of the
shuttle); tank mass in the boosters is cheap by comparison.
Any crossfeed system does complicate the plumbing, though. For the shuttle,
almost certainly it would be avoided, to minimize changes to the existing
hardware.
--
When failure is not an option, success | Henry Spencer henry@spsystems.net
can get expensive. -- Peter Stibrany | (aka henry@zoo.toronto.edu)

Newsgroups: sci.space.tech
From: henry@spsystems.net (Henry Spencer)
Subject: Re: 2 fuels, 1 oxydizer SSTO...
Date: Tue, 19 Dec 2000 13:02:47 GMT
In article <3A3E7B6D.A19F4795@GramlichXYZ.NetXYZ>,
Wayne Gramlich <WayneXYZ@GramlichXYZ.NetXYZ> wrote:
>I'm trying to figure out why "larger tanks need thicker walls to
>carry the same pressure loads"...
See my other posting about slicing.
>My thought experiment is to take a cylindrical tank and chop it in
>half at the middle. It seems to me that it should be possible to
>add an arbitrary amount of length between the two halves without
>having to make either of the two ends any thicker (as long as the
>pressure is kept constant.) What am I missing here?
Nothing, except that this particular approach doesn't help the tank-wall
area situation: the wall area grows linearly with the added volume if you
only expand the tank in one dimension. The superficial attractiveness of
larger tanks comes from expansion in two or three dimensions, which does
reduce wall area... but also needs thicker walls.
>I had always thought that tank thickness on rockets was dominated by
>weight loads due to gravity and launch acceleration (e.g. the tank has
>to handle the weight of the stages/payload above it and the fuel weight.)
No, inertia loads are problematic only for very large rockets (although
hydraulic head in the tanks contributes to the pressure load). Even the
modest pressurization needed for suppressing cavitation in pumps supplies
plenty of structural strength. Remember that 1atm pressure is ten tons
per square meter -- that's a lot.
--
When failure is not an option, success | Henry Spencer henry@spsystems.net
can get expensive. -- Peter Stibrany | (aka henry@zoo.toronto.edu)